240 COLOUR IN NATURE chap. 



nective tissue cells of the body cavity, and migrates 

 outwards through the blood-vessels in the wandering 

 cells which fill up the cavities of these, or whether it 

 may arise in part in the epidermis in situ, and con- 

 fines himself to emphasising the fact of the relation 

 between the pigment spots and the obliterated open- 

 ings of the transverse vessels. When we remember, 

 however, that the places where blood stagnates are 

 especially liable to become deeply pigmented, it is diffi- 

 cult to avoid the conclusion that the pigment directly 

 originates from the degenerating blood which must 

 accumulate at these points. The question has some 

 bearing upon theories of the origin of colour and 

 colour patterns, but is obscured by the usual diffi- 

 culty of distinguishing between post and propter. 



In connection with this paper of Zenneck's we 

 may note an observation by Mr. J. Loeb on the rela- 

 tion between the blood-vessels and the coloration of 

 the yolk-sac in the embryos of the fish Fundulus. 

 The yolk-sac has here a peculiarly tiger-like colour 

 due to a combination of black and red chromato- 

 phores. At the time of their first appearance the 

 pigment cells are practically uniformly scattered, 

 but as the blood-vessels develop and the blood 

 begins to circulate, the chromatophores begin to 

 migrate to the surface of the vessels, and ultimately 

 pigment is only visible as a covering to the vessels. 

 By an ingenious experiment with a heart poison 

 (potassium chloride), Loeb convinced himself that 

 the migration only occurred when the blood was 

 actually circulating in the vessels of the embryo. 

 He concludes that the coloration of the yolk-sac 

 in Fundulus is due to a specific irritability of the 



